The present invention relates to the creation of artificial turf playing surface.
Artificial playing fields have gained almost universal preference over natural dirt and grass playing fields because of the reliability and ease of maintenance. Two concerns associated with artificial playing fields are player safety and uniformity of the playing surface. The artificial surface needs to allow natural movement of a player""s feet when in contact with the surface to prevent sprains, tearing of tendons and muscles, and to prevent broken bones. Moreover, the artificial surface needs to prevent burns to exposed skin of the players caused by players sliding along the artificial surface. In addition, the field must have resilience to absorb some of the shock of player contact. Uniformity of the playing surface is important so that the field does not develop areas that are less resilient than others or areas where the depth of the playing surface varies from one location to another. Lack of uniformity affects both safety and performance as the soccer, football or rugby ball in use will bounce or roll differently depending on the field characteristics. Finally, it is desirable to construct an artificial field in which drainage can be achieved without a raised crown in the center of the field. The raised crown makes ball handling different from a flat natural surface because a player kicking or throwing a ball from the center of the field will be higher than a receiving player near the outer edge of the field.
Artificial turf is presently made of polyurethane mats with artificial grass made of polyethylene or nylon stitched to the mats which are then laid on a field. One method of improving appearance, performance and safety is to fill in the spaces between the artificial grass with an infil material to make the blades stand up straight. As used herein, infil material means particles that can be poured and raked into and between blades, shafts or fibers of artificial grass. The most common infil materials are sand and resilient particles such as rubber granules. The infil material is poured over the mats and raked into the artificial grass until only the top ends of the grass are showing. Problems arise because the various infil materials, individually or in mixtures, will migrate horizontally and vertically. Horizontal migration occurs when player movement kicks up infil material. Additionally, rain will cause infil material to rise and move with the flow of water over the field, particularly with fields designed with a center crown. Vertical migration occurs when small or fine particles move downward within the infil material and the larger particles move to the top. Vertical migration is caused by vibration or player movement and contact. Vertical migration is also caused by rain water draining down through the infil material and causing the infil material to float and move with the movement of the water. A method, known as the thatch system, combats migration by tufting every other stitch with textured nylon in an attempt to limit movement of the infil material.
The prior art discloses numerous attempts to improve the safety and performance of infil material and to minimize or eliminate vertical and horizontal migration of the infil material. U.S. Pat. No. 5,580,708 discloses a fibrillated fiber. FIG. 1 depicts a representative synthetic grass fiber 36 which is fibrillated. Fibrillated as used in the ""708 patent means that synthetic grass fibers 36 have several blade openings 40 through each synthetic grass fiber 36. Blade openings 40 result in connected strands 38 within synthetic grass fiber 36. U.S. Pat. No. 4,337,283 discloses a mixture of sand and rubber particles in order to achieve resiliency of the field playing surface. U.S. Pat. No. 5,976,645 (the ""645 patent) discloses a system comprising a pile fabric over a porous aggregate layer. The pile elements are tufted to a backing and an infil layer is introduced consisting essentially of resilient particles. The ""645 patent discloses infil particles which can be natural rubber, synthetic rubber such as styrene butadine (ground tire rubber), butyl rubber, neoprene, urethane rubber and nitrile rubber. U.S. Pat. No. 5,958,527 (the ""527 patent) discloses a pure sand base, a pure rubber top course, and a middle course of mixed sand and rubber. The ""527 patent also discloses fibrillated top ends of the artificial grass to retain the relatively large top rubber particles in a loose netlike flexible structure. The loose criss-cross net of fibrillated fibers also allows dislodged rubber particles to work back into the underlying top rubber course.
U.S. Pat. No. 4,396,653 discloses fibrillated fibers, rubber particles and a binder material that may be added to the rubber like particles. FIG. 2 depicts the prior art configuration of the ""563 patent. Artificial turf 10 has fibers 14 woven into base sheet 20 with warp type strand 16 and woof type strand 18. Fibers 14 are fibrillated to produce separate fiber ends 12. The binders disclosed are commercially available rubber adhesives or cements which are sprayed or spilled upon infil particles in lower layer 24 so as to trickle down between and connect adjacent particle surfaces that are contacted by the adhesive or cement. The binder does not fill the spaces between the particles, but serves to tack adjacent portions of particle surfaces to one another forming a porous material in which at least some, but not necessarily all, of the particles are spot fastened together. FIG. 2 depicts a representation of binder 34 tacking portions 36 of particles 32 to binder 34.
U.S. Pat. No. 4,735,825 (the ""825 patent) discloses a bonding agent which can either be permanent or temporary. The ""825 patent discloses mixing the bonding agent with the free flowing bulk material prior to the material being distributed in the pile of the artificial grass and activating the bonding agent at a later time. The ""825 patent discloses a heat treatment for activating the bonding agent. The ""825 patent discloses such water insoluble agents as PVC powder, PET powder, melting fibers, aqueous synthetic resin dispersing, or synthetic resin dispersing in solvent for bonding agents. In the ""825 patent, drainage of the bonded infil material is achieved by mixing a water soluble element with the rubber granules and the bonding agent so that when water is added the water soluble element will wash out leaving spaces for drainage.
What is needed beyond the prior art is a way to bond rubber particles by physical adhesion so that drainage will take place in the spaces between the granules and also to achieve uniform results in adhesion. Additionally, what is needed is a way to bond a layer of rubber granules near the mat to prevent vertical and horizontal migration so that there will always be a minimum layer of rubber to absorb impact while allowing a loose upper layer of granules to improve field performance. Finally, what is needed beyond the prior art is a way to prevent horizontal migration of the upper layer of loose rubber granules. The goal therefore, is to achieve an infil system that will not migrate or displace over a long period of time.
The invention meeting the needs identified above is an artificial turf field comprising a mat with artificial grass stitched to the mat and rubber granules combined with powdered latex or other suitable powdered binding agent activated by water. Mats with the artificial grass affixed are laid on an athletic field. Rubber granules mixed with a bonding agent and the coated rubber granules are poured over the mats and raked until a depth of xc2xd to xc2xe inches is achieved. Water is then misted onto the surface activating the bonding agent. The first layer is allowed to set. The first level of infil material will be bound particle to particle, to the grass and to the mat by the agent, thus ensuring a stablized shock absorbency pad. Additionally, the stabilized layer eliminates substantial migration of loose infil material. Next, uncoated rubber granules are applied to the field and raked in until only the top of the grass shows providing a finished appearance, the feel of a playing surface that is uniform, and a surface that is prevented from becoming thicker and harder in various places due to migration. The artificial grass fibers are brushed to fibrillate the ends of the artificial grass fibers.